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188 lines
4.9 KiB
C
188 lines
4.9 KiB
C
/* flonum_mult.c - multiply two flonums
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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This file is part of GAS, the GNU Assembler.
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GAS is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3, or (at your option)
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any later version.
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GAS is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
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License for more details.
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You should have received a copy of the GNU General Public License
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along with GAS; see the file COPYING. If not, write to the Free
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Software Foundation, 51 Franklin Street - Fifth Floor, Boston, MA
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02110-1301, USA. */
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#include "ansidecl.h"
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#include "flonum.h"
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/* plan for a . b => p(roduct)
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+-------+-------+-/ /-+-------+-------+
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| a | a | ... | a | a |
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| A | A-1 | | 1 | 0 |
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+-------+-------+-/ /-+-------+-------+
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+-------+-------+-/ /-+-------+-------+
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| b | b | ... | b | b |
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| B | B-1 | | 1 | 0 |
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+-------+-------+-/ /-+-------+-------+
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+-------+-------+-/ /-+-------+-/ /-+-------+-------+
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| p | p | ... | p | ... | p | p |
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| A+B+1| A+B | | N | | 1 | 0 |
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+-------+-------+-/ /-+-------+-/ /-+-------+-------+
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/^\
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(carry) a .b ... | ... a .b a .b
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A B | 0 1 0 0
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... | ... a .b
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| 1 0
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| ...
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| ___
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| \
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+----- P = > a .b
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N /__ i j
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N = 0 ... A+B
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for all i,j where i+j=N
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[i,j integers > 0]
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a[], b[], p[] may not intersect.
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Zero length factors signify 0 significant bits: treat as 0.0.
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0.0 factors do the right thing.
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Zero length product OK.
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I chose the ForTran accent "foo[bar]" instead of the C accent "*garply"
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because I felt the ForTran way was more intuitive. The C way would
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probably yield better code on most C compilers. Dean Elsner.
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(C style also gives deeper insight [to me] ... oh well ...) */
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void
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flonum_multip (const FLONUM_TYPE *a, const FLONUM_TYPE *b,
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FLONUM_TYPE *product)
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{
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int size_of_a; /* 0 origin */
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int size_of_b; /* 0 origin */
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int size_of_product; /* 0 origin */
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int size_of_sum; /* 0 origin */
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int extra_product_positions; /* 1 origin */
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unsigned long work;
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unsigned long carry;
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long exponent;
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LITTLENUM_TYPE *q;
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long significant; /* TRUE when we emit a non-0 littlenum */
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/* ForTran accent follows. */
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int P; /* Scan product low-order -> high. */
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int N; /* As in sum above. */
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int A; /* Which [] of a? */
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int B; /* Which [] of b? */
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if ((a->sign != '-' && a->sign != '+')
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|| (b->sign != '-' && b->sign != '+'))
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{
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/* Got to fail somehow. Any suggestions? */
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product->sign = 0;
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return;
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}
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product->sign = (a->sign == b->sign) ? '+' : '-';
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size_of_a = a->leader - a->low;
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size_of_b = b->leader - b->low;
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exponent = a->exponent + b->exponent;
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size_of_product = product->high - product->low;
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size_of_sum = size_of_a + size_of_b;
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extra_product_positions = size_of_product - size_of_sum;
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if (extra_product_positions < 0)
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{
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P = extra_product_positions; /* P < 0 */
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exponent -= extra_product_positions; /* Increases exponent. */
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}
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else
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{
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P = 0;
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}
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carry = 0;
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significant = 0;
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for (N = 0; N <= size_of_sum; N++)
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{
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work = carry;
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carry = 0;
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for (A = 0; A <= N; A++)
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{
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B = N - A;
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if (A <= size_of_a && B <= size_of_b && B >= 0)
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{
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#ifdef TRACE
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printf ("a:low[%d.]=%04x b:low[%d.]=%04x work_before=%08x\n",
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A, a->low[A], B, b->low[B], work);
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#endif
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/* Watch out for sign extension! Without the casts, on
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the DEC Alpha, the multiplication result is *signed*
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int, which gets sign-extended to convert to the
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unsigned long! */
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work += (unsigned long) a->low[A] * (unsigned long) b->low[B];
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carry += work >> LITTLENUM_NUMBER_OF_BITS;
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work &= LITTLENUM_MASK;
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#ifdef TRACE
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printf ("work=%08x carry=%04x\n", work, carry);
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#endif
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}
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}
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significant |= work;
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if (significant || P < 0)
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{
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if (P >= 0)
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{
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product->low[P] = work;
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#ifdef TRACE
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printf ("P=%d. work[p]:=%04x\n", P, work);
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#endif
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}
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P++;
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}
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else
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{
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extra_product_positions++;
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exponent++;
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}
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}
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/* [P]-> position # size_of_sum + 1.
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This is where 'carry' should go. */
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#ifdef TRACE
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printf ("final carry =%04x\n", carry);
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#endif
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if (carry)
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{
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if (extra_product_positions > 0)
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product->low[P] = carry;
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else
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{
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/* No room at high order for carry littlenum. */
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/* Shift right 1 to make room for most significant littlenum. */
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exponent++;
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P--;
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for (q = product->low + P; q >= product->low; q--)
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{
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work = *q;
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*q = carry;
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carry = work;
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}
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}
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}
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else
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P--;
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product->leader = product->low + P;
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product->exponent = exponent;
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}
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